The pathogenesis of autoimmunity in New Zealand mice is still uncertain; however, advances are being made. Genetic, viral and immunologic factors appear to be involved. The immunologic factors are complex. A very early defect is a loss of thymic regulatory or suppressor cells. These cells may keep the immune system in check and when deficient allow autoimmune responses to occur. We are trying the separate and characterize these cells with an aim toward understanding the precise cellular defect. Antibodies to T cells are produced by New Zealand mice. The role of these antibodies in the loss of suppressor cells is being evaluated by several techniques. Administration of these anti-T cell antibodies to NZB/NZW mice from 3 days to 1 month of age preferentially eliminates suppressor cells. BIBLIOGRAPHIC REFERENCES: Raveche, E.S., Klassen, L.W. and Steinberg, A.D.: Sex differences in the formation of anti-T-cell antibodies. Nature 263: 415-416, 1976. Eilat, E., DiNatale, P., Steinberg, A.D. and Schechter, A.N.: Properties of tRNA specific antibodies from NZB/NZW mice. J. Immunol 118: 1016-1024, 1977.